1. Field of the Invention
The present invention relates to an apparatus for restarting an inverter for driving a synchronous motor after momentary interruption, and particularly to an apparatus which restarts inverters when a power supply is restored after a momentary power outage in spinning lines or the like, wherein synchronous motors are operated in groups by the inverters.
2. Description of Related Art
A conventional restart method applied to recovery after a momentary interruption usually stops the output of an inverter when the power supplied to the inverter is lost, and then recovers the output when the power is subsequently restored. To begin the restart operation when the power is restored, the output frequency of the inverter is set at the same frequency as that when the inverter is stopped, and only the output voltage is gradually increased.
In the conventional method, the output voltages of the inverter at the restart will be out of phase with the voltages at the motor terminals because the restart timing is set at random.
As a result, the rush current at the restart may become large. This crests a problem in that the capacity and installation area of the inverter become large, and its cost increases.
In addition, when the speed change in the motor is great owing to a heavy load, tedious steps as follows must be taken in restoring the inverter. First, the output frequency of the inverter is gradually reduced by a current limiting operation after the restart until the speed of the motor matches the output frequency, and then, the motor is reaccelerated to the original speed.
This poses another problem in that the restart time is prolonged because it takes a lot of time for the motor to restore its original speed.